In the related art, a Fabry-Perot laser element (FP laser element) using a Fabry-Perot resonator is known as a semiconductor laser element. Since stability of an oscillation mode in this FP laser element is low, a DFB semiconductor laser element has been proposed as an improved semiconductor laser element. In the DFB semiconductor laser element, a diffraction grating is provided inside a resonator, and laser light with a wavelength component selected by the diffraction grating is amplified.
In a DFB semiconductor laser element described in Japanese Unexamined Patent Publication No. 9-283837, a barrier layer is disposed between well layers with different thicknesses in an active layer having a multiple quantum well (MQW) structure, and the layers are formed of InGaAsP.
In an InGaAsP-based DFB semiconductor laser element described in Japanese Patent No. 2647018, there is a problem in that, when an operation temperature is changed, a gain peak fluctuates due to dependence of a band gap energy of a semiconductor on temperature, and a Bragg wavelength fluctuates due to dependence of a refractive index of the semiconductor on temperature.
Therefore, in Japanese Patent No. 2647018, quantum well layers having different thicknesses are alternately arranged and a plurality of gain spectra corresponding to a group of quantum well layers with respective thicknesses are set to reduce dependence of an optical gain peak wavelength on temperature.
In an InGaAsP-based DFB semiconductor laser element described in Japanese Unexamined Patent Publication No. Hei 7-249829, similarly, film thicknesses of a plurality of quantum well layers are changed.
As a distance from the diffraction grating decreases, the film thicknesses of the well layers increase, an emission wavelength increases, and dependence of an optical gain peak wavelength on temperature is reduced.